In my field (microbiology/bioinformatics), it's almost impossible to find pre-prints of pay walled papers and emailing the authors and hoping that they will respond at all also doesn't seem an extremely efficient process for literature research.
The other thing that may only be localised to my field is that every author recognizes this problem and makes a copy available on their website or university's working paper site. I would only rarely resort to going to an actual journal because Google Scholar was way easier to find a copy of the article.
This is, of course, going to differ for a number of fields and there is a growing trend for authors and journals to open up their articles.